RHOMBENCEPHALOSYNAPSIS

Figure 1: Coronal T2 image showing fused dentate nucleus

Figure 2: Coronal T2 weighted image showing typical transverse folia and sulci with fused cerebellar hemispheres

Figure 3: Axial T2WI showing fusion of the interfoliate sulci across midline along with fusion of cerebellar folia.

Figure 4: Axial T2WI showing "keyhole appearanca" of 4th ventricle and midline fusion of cerebellar white matter. Vermis is not seen.

Figure 5: Sagittal T2WI showIng absent primary fissure. There is also slight rounding of the fastgial recess. Note the corpus callosum in a patient of isolated rhombencephalosynapsis.

The above MR was done in an adult patient presenting with mild gait ataxia. Rhombencephalosynapsis was detected in him as an isolated finding.

• Rhombencephalosynapsis is a rare  posterior fossa malformation characterized by aplastic/ hypoplastic cerbellar vermis and fusion of cerebellar hemispheres in the midline.
• It was forst described by Obersteiner in 1914.
• Pathogenesis is related to disturbed cerebellar development between 28 to 41 days of gestation.
• Clinical profile varies and is related to the posterior fossa malformation and associated supratentorial anomalies. It might vary from mild truncal ataxia and normal intelligence to cerebral palsy.
• Isolated rhombencephalosynapsis is less common than rhombencephalosynapsis associated with supratentorial anomalies
• Most cases are reported in infancy or pediatric age group with only few cases reported in adults.

Diagnosis is usually by MRI which is clearly superior to CT in evaluating these patients.

Distinctive radiological features are:
Posterior fossa:

• Agenesis / hypoplastic vermis 
• Absent posterior cerebellar incisura, vallecula
• agenesis or hypogenesis of anterior vermis ( rostral ) with hypoplastic posterior vermis( caudal  nodulus may form
• Midline fusion of cerebellar hemispheres to form a single lobed cerebellum
• Fissures and cerebellar folia are transversely oriented best seen on coronal MRI
• Superior and middle cerebellar peduncles may be fused along with dentate nuclei giving a characeteristic diamond shaped, posteriorly pointing 4th ventricle

Supratentorial anomalies:

• Most common supratentorial anomaly is hydrocephalus with aqueductal stenosis
• Sometimes, there may be fused thalami, fornIces, cerebral peduncles, inferior collicluli with absent septum pellucidum
• Dysgenetic limbic system
• cortical malformatins
• sutural synostosis
• hypoplastic commissures, hypoplastic anterior visual pathway and agenesis of posterior lobe of pituitary gland
• Association with septo-optic dysplasia and holoprosencephaly

 References:

• Mendonca JLF, Natal MRCR, Viana SL et al. Rhombencephalosynapsis: CT and MRI findings. Neurology India;152:118-120, 2004.
• Weerakkody Y, Gaillard F et al. Rhombencephalosynapsis. Radiopaedia.org.
• Blaser SI: Rhombencephalosynapsis. Diagnostic Imaging Brain: First Edition. Amirsys: I-1-30, 2005.